1. Field of the Invention
The present invention relates to semiconductor-based solar cells. In particular, the present invention relates to solar cells including integrated semiconductor structures with back-contact cell design and to methods of manufacturing such cells.
2. Description of Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power.
Thin-film technology has gained widespread interest in satellite and in general space applications because of their low cost, high-density packaging, and radiation resistance. Thin-film solar cells can be in general based on different semiconductors materials, such as polycrystalline or amorphous silicon, binary to quaternary alloys of III-V semiconductors, CdTe— or copper-indium-gallium selenide (CIGS) based materials. A key issue in the development of thin-film solar cells for space applications has been the production of high-power density solar modules to be used as the primary power source for spacecraft.
Solar cells are often fabricated in single junctions or in vertical multi-junction structures, and arranged in horizontal arrays, where the individual cells are series-connected one to another.
Inverted metamorphic solar cell structures based on Ill-V compound semiconductor layers, such as described in M. W. Wanlass et al., “Lattice Mismatched Approaches for High Performance”, III-V Photovoltaic Energy Converters (Conference Proceedings of the 31st IEEE Photovoltaic Specialists Conference, Jan. 3-7, 2005, IEEE Press, 2005), present an important conceptual starting point for the development of future commercial high efficiency solar cells.
When a solar cell is fabricated on a vertical multi-junction structure, one electrical contact is usually placed on the front side of the cell (the side exposed to solar radiation), while the other electrical contact is placed on the back side of the solar cell (the side not facing the sun). Designs in which both electrical contacts are located on one side of the solar cell have recently gained particular attention since they can optimize interconnection between cells. Solar cells having both electrical contacts located on the backside of the cell are often referred to as back-contact solar cells.
US patent application No. 2008/0185038 describes a multi-junction solar cell, and in particular an inverted metamorphic solar cell, with both anode and cathode contacts on the backside of the cell. A sequence of layers of semiconductor material is deposited on a first substrate, the sequence of layers forming at least one cell of a multi-junction solar cell; a via is etched through the sequence of layers from the top surface of the layers to the first substrate; a second substrate is provided over the sequence of layers; and the first substrate is removed. A first electrode is then formed on the back side of the solar cell and an electrical connection is formed between the top cell of the multi-junction solar cell and a second electrode on the back side of the solar cell.
U.S. patent application Ser. No. 12/190,449, filed on Aug. 12, 2008 in the name of the instant assignee, discloses a method of manufacturing a solar cell by providing a first semiconductor substrate; depositing on the first substrate a sequence of layers of semiconductor material forming a solar cell including a metal contact layer; mounting a surrogate second substrate on top of the sequence of layers; removing the first substrate; simultaneously etching first and second troughs around the periphery of said solar cell, wherein the first trough lies within the second trough so as to form first and second mesa structures on the surrogate second substrate; and etching said second trough down to said metal contact layer.